From: Duy Nguyen <pclouds@gmail.com>
To: David Turner <dturner@twopensource.com>
Cc: Git Mailing List <git@vger.kernel.org>,
Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH v2 03/17] index-helper: new daemon for caching index and related stuff
Date: Tue, 29 Mar 2016 09:31:39 +0700 [thread overview]
Message-ID: <CACsJy8Bk19NNacDwez6BzicThnVUDQEoGe3m+ThiorP8uP9+eA@mail.gmail.com> (raw)
In-Reply-To: <1458349490-1704-4-git-send-email-dturner@twopensource.com>
On Sat, Mar 19, 2016 at 8:04 AM, David Turner <dturner@twopensource.com> wrote:
> From: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
>
> Instead of reading the index from disk and worrying about disk
> corruption, the index is cached in memory (memory bit-flips happen
> too, but hopefully less often). The result is faster read. Read time
> is reduced by 70%.
>
> The biggest gain is not having to verify the trailing SHA-1, which
> takes lots of time especially on large index files. But this also
> opens doors for further optimiztions:
>
> - we could create an in-memory format that's essentially the memory
> dump of the index to eliminate most of parsing/allocation
> overhead. The mmap'd memory can be used straight away. Experiment
> [1] shows we could reduce read time by 88%.
This reference [1] is missing (even in my old version). I believe it's
http://thread.gmane.org/gmane.comp.version-control.git/247268/focus=248771,
comparing 256.442ms in that mail with v4 number, 2245.113ms in 0/8
mail from the same thread.
> Git can poke the daemon via named pipes to tell it to refresh the
> index cache, or to keep it alive some more minutes. It can't give any
> real index data directly to the daemon. Real data goes to disk first,
> then the daemon reads and verifies it from there. Poking only happens
> for $GIT_DIR/index, not temporary index files.
I think we should go with unix socket on *nix platform instead of
named pipe. UNIX named pipe only allows one communication channel at a
time. Windows named pipe is different and allows multiple clients,
which is the same as unix socket.
> $GIT_DIR/index-helper.pipe is the named pipe for daemon process. The
> daemon reads from the pipe and executes commands. Commands that need
> replies from the daemon will have to open their own pipe, since a
> named pipe should only have one reader. Unix domain sockets don't
> have this problem, but are less portable.
Hm..NO_UNIX_SOCKETS is only set for Windows in config.mak.uname and
Windows will need to be specially tailored anyway, I think unix socket
would be more elegant.
> +static void share_index(struct index_state *istate, struct shm *is)
> +{
> + void *new_mmap;
> + if (istate->mmap_size <= 20 ||
> + hashcmp(istate->sha1,
> + (unsigned char *)istate->mmap + istate->mmap_size - 20) ||
> + !hashcmp(istate->sha1, is->sha1) ||
> + git_shm_map(O_CREAT | O_EXCL | O_RDWR, 0700, istate->mmap_size,
> + &new_mmap, PROT_READ | PROT_WRITE, MAP_SHARED,
> + "git-index-%s", sha1_to_hex(istate->sha1)) < 0)
> + return;
> +
> + release_index_shm(is);
> + is->size = istate->mmap_size;
> + is->shm = new_mmap;
> + hashcpy(is->sha1, istate->sha1);
> + memcpy(new_mmap, istate->mmap, istate->mmap_size - 20);
> +
> + /*
> + * The trailing hash must be written last after everything is
> + * written. It's the indication that the shared memory is now
> + * ready.
> + */
> + hashcpy((unsigned char *)new_mmap + istate->mmap_size - 20, is->sha1);
You commented here [1] a long time ago about memory barrier. I'm not
entirely sure if compilers dare to reorder function calls, but when
hashcpy is inlined and memcpy is builtin, I suppose that's possible...
[1] http://article.gmane.org/gmane.comp.version-control.git/280729
> +}
--
Duy
next prev parent reply other threads:[~2016-03-29 2:32 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-19 1:04 [PATCH v2 00/15] index-helper, watchman David Turner
2016-03-19 1:04 ` [PATCH v2 01/17] read-cache.c: fix constness of verify_hdr() David Turner
2016-03-19 1:04 ` [PATCH v2 02/17] read-cache: allow to keep mmap'd memory after reading David Turner
2016-03-19 1:04 ` [PATCH v2 03/17] index-helper: new daemon for caching index and related stuff David Turner
2016-03-29 2:31 ` Duy Nguyen [this message]
2016-03-29 21:51 ` David Turner
2016-03-19 1:04 ` [PATCH v2 04/17] index-helper: add --strict David Turner
2016-03-19 1:04 ` [PATCH v2 05/17] daemonize(): set a flag before exiting the main process David Turner
2016-03-19 1:04 ` [PATCH v2 06/17] index-helper: add --detach David Turner
2016-03-29 2:35 ` Duy Nguyen
2016-03-19 1:04 ` [PATCH v2 07/17] read-cache: add watchman 'WAMA' extension David Turner
2016-03-19 1:04 ` [PATCH v2 08/17] read-cache: invalidate untracked cache data when reading WAMA David Turner
2016-03-29 2:50 ` Duy Nguyen
2016-03-29 21:22 ` David Turner
2016-03-19 1:04 ` [PATCH v2 09/17] Add watchman support to reduce index refresh cost David Turner
2016-03-24 13:47 ` Jeff Hostetler
2016-03-24 17:58 ` David Turner
2016-03-19 1:04 ` [PATCH v2 10/17] index-helper: use watchman to avoid refreshing index with lstat() David Turner
2016-03-19 1:04 ` [PATCH v2 11/17] update-index: enable/disable watchman support David Turner
2016-03-19 1:04 ` [PATCH v2 12/17] unpack-trees: preserve index extensions David Turner
2016-03-19 1:04 ` [PATCH v2 13/17] index-helper: kill mode David Turner
2016-03-19 1:04 ` [PATCH v2 14/17] index-helper: don't run if already running David Turner
2016-03-19 1:04 ` [PATCH v2 15/17] index-helper: autorun mode David Turner
2016-03-19 1:04 ` [PATCH v2 16/17] index-helper: optionally automatically run David Turner
2016-03-19 1:04 ` [PATCH v2 17/17] read-cache: config for waiting for index-helper David Turner
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